Hitherto SiO2 has been continued to be used for a gate oxide film in semiconductor production for many years. The reason for this is that speeding-up of the operating speed of an element with upgraded integration of semiconductor could be dealt with by promoting film-thinning of SiO2. However, in these years, as a result of the fact that speeding-up of the operation speed of the element is further promoted in order to attain upgrading of functionality and integration of LSI, a physical limit for film-thinning of SiO2 has come near and therefore it has become difficult to deal with further speeding-up. In view of this, attention has been paid on a hafnium-containing oxide film as a gate oxide film in place of SiO2. The hafnium-containing oxide film is high severalhold in dielectric constant as compared with SiO2, and therefore it becomes possible to increase a physical film thickness corresponding to speeding-up of operational speed of the element as compared with that of SiO2.
As film formation process for this hafnium-containing oxide film, there are two kinds, a physical vapor-phase growth process (Physical Vapor Deposition, abbreviated hereinafter as PVD) and a chemical vapor-phase growth process (Chemical Vapor Deposition, abbreviated hereinafter as CVD). In general, with PVD process, forming a uniform film on a substrate having unevenness is difficult, and controlling a film composition is difficult. In contrast, with CVD process, formation of a uniform film on a substrate is possible regardless of presence or absence of unevenness, and controllability for a film composition is also excellent. In film formation of the gate oxide film, there is a case that formation of a uniform film on a part having unevenness is required, according to process of gate stack production. Additionally, it becomes important to control the film composition because the film composition affects the electrical properties of semiconductor. Therefore, it has become a main stream to use CVD process as a film formation process for the gate oxide film.
In order to carry out the film formation of a hafnium-containing oxide film by CVD process, a hafnium film formation raw material high in vapor pressure is required, in which hafnium amide complex is relatively high in volatility and expanding in use as a film formation raw material for the hafnium-containing oxide film as a semiconductor gate oxide film by CVD process. As a production process for the hafnium amide complex, a process for allowing hafnium tetrachloride (HfCl4) and lithium alkylamide to react each other in an organic solvent is general.
For example, a process for allowing hafnium tetrachloride (HfCl4) and lithium diethylamide [LiN(CH2CH3)2] to react each other in a hexane solvent to synthesize tetrakis(diethylamido)hafnium {Hf[N(CH2CH3)2]4} is disclosed (see Non-patent Citation 1).
The gate oxide film is required to be a film having an extremely high purity because of being positioned at a lower-most layer section of semiconductor, and therefore the hafnium amide complex as the film formation raw material therefor is also required to be a high purity product. A zirconium component originated from a raw material, of impurities contained in the hafnium amide complex is contained usually at a high concentration of about 1000 to 5000 mass ppm. This is because hafnium and zirconium are the same group elements and have similar chemical properties under lanthanoid contraction so as to be very difficult to be separated from each other.
It is pointed out that a zirconium oxide is low in heat resistance as compared with a hafnium oxide, and the zirconium oxide is taken into a film depending upon a zirconium impurity concentration in a hafnium film formation raw material, thereby raising contingent trouble of a device. Thus, it is required to lower a zirconium concentration in the film. Accordingly, using a hafnium film formation raw material lowered in zirconium concentration is important for ensuring a reliability of the device (see Non-patent Citation 2).
As production method for a hafnium film formation raw material lowered in zirconium concentration, there are a production method for hafnium amide complex by a reduced-pressure rectification (see Patent Citation 1 and Patent Citation 2), a method of production by light irradiation or by passing through a chelate-carrying column (see Patent Citation 3), a method of producing a hafnium film formation raw material after carrying out a recrystallization for hafnium halide with an organic solvent having ether linkage (see Patent Citation 4), a production method for hafnium amide complex by carrying out a reduced-pressure distillation after additive such as CF3SO3H or the like is added to hafnium amide complex (see Patent Citation 5), and the like. For example, Japanese Patent Provisional Publication No. 2005-298467 (Patent Citation 2) describes a method of separating zirconium component in tetrakis(dimethylamido)hafnium by a reduced-pressure rectification, in which ligand of hafnium amide complex is limited to tetrakis(dimethylamido)hafnium having dimethylamino group so that application cannot be made to hafnium amide complex such as tetrakis(ethylmethylamido)hafnium or tetrakis(diethylamido)hafnium.
Additionally, in Japanese Patent Provisional Publication No. 2005-314785 (Patent Citation 3) and Japanese Patent Provisional Publication No. 2007-051042 (Patent Citation 4), there is no description of yield and yield point of hafnium amide complex so that it is not apparent as to whether execution is economically possible or not.